A key House subcommittee yesterday voted to bar the administration from putting into effect a set of major rules changes that would drastically alter the way the government's 1.4 million white-collar workers are picked for raises, promotions or layoffs.

If Congress goes along, the proposal would block, at least until October, implementation of the White House plan to devalue seniority and put greater emphasis on performance.

Length of service is now the determining factor in getting a within-grade pay raise, or avoiding a RIF.

Federal worker groups have protested the Reagan rules change, due to go into effect later this year.

Employe unions contend that basing an employe's eligibility for a raise, promotion or layoff on his or her most recent performance rating would give political bosses a strangle hold on the career civil service. They say appointees could, and would, withhold the within-grade pay raises (which are now virtually automatic every one to three years) by giving nonfavorites a single bad rating.

Unions also argue that the proposed RIF rules change would make it too easy for political appointees to clean out pockets of competent, but politically uncooperative, career people by giving them bad ratings and then running a RIF.

Since the Reagan administration took office, about 2,900 federal workers have been fired for economy reasons. They were let go under the existing last-hired-first-fired system, which the administration says punishes outstanding employes who lack seniority, and especially hurts women and minorities.

Language to block the rules changes was approved yesterday by the House subcommittee on Treasury-Postal Service-General Government. The language, introduced by Rep. Steny Hoyer (D-Md.) is contained in a supplemental appropriation to fund operations through Oct. 1, the start of the new fiscal year. All four Democrats present voted with Hoyer to block the rules change. All three Republican members voted against it.

Hoyer's plan puts a hold on any rules changes until October. It also directs the General Accounting Office (an arm of Congress) to study and comment on the proposed changes.

Hoyer will try to get a similar rules-change ban inserted into appropriations bills for the upcoming fiscal year.